Many electronic products are provided in the form of a plurality of components, such as, electrical circuit elements and/or integrated circuits, mounted upon an underlying substrate, wherein that substrate provides various conductive paths between two or more of the mounted components, and/or between at least one of the mounted components and an external connection terminal. One well known form of substrate for mounting components and for providing the aforementioned conductive paths is the printed circuit board.
Printed circuit board manufacturing generally involves the formation of a plurality of conductive traces disposed on at least one major surface of a generally planar, generally rigid, insulating material. Many insulating materials have been used to form substrates for electronic products, such as, but not limited to, FR4, epoxy, and ceramics. Printed circuit boards may have conductive traces disposed on each of two opposing sides thereof. Similarly, printed circuit boards may have one or more layers of conductive traces disposed within the insulating material. Printed circuit boards may have holes, or openings, therethrough for the mounting of components, mechanical alignment of the printed circuit board to another unit, or plated through holes for the interconnection of the various layers of conductive traces.
A substrate similar to the printed circuit board, but which is not generally rigid, is referred to herein as a flex substrate. Such flex materials with patterned conductive traces are well known in the electronic arts and have been used for a variety of applications including backplane connector cables, and for providing a base upon which components may be attached.
A common manufacturing process for forming conductive traces on printed circuit boards, includes forming a blanket layer of conductive material and then etching away certain portions so as to form spaces between the remaining conductive material which, subsequent to this etching operation, is in the form of conductive traces. A commonly used conductive material for such applications is copper.
In certain circumstances, conductive material with chemical and electrical properties different from those of copper are required. One conductive material that is used in such circumstances is gold. Gold has a lower electrical resistivity than copper, and this lower electrical resistivity provides for improved electrical performance in many types of electrical circuits. Additionally, gold, as compared to copper, has the desirable property of being less reactive to atmospheric gases, and hence is less likely to corrode.
Those skilled in the electronic arts in general, and in circuit board design and manufacturing, and integrated circuit packaging, in particular, will recognize that although gold has certain desirable electrical and chemical properties, it is an expensive element to include in a product or in a manufacturing process. In addition to being expensive, some manufacturing processes for applying gold require a wet chemical environment.
What is needed are methods and apparatus that are convenient and cost-effective for applying gold to portions of substrates, such as, circuit boards and integrated circuit packages.